Web crawler scheduler that utilizes sitemaps from websites

ABSTRACT

Systems and methods for scheduling documents for crawling are disclosed. In some implementations, a method includes obtaining sitemap information for a plurality of websites; and analyzing the sitemap information to identify a website, in the plurality of websites. The website has sitemap information that is at least potentially out of date. The method also includes updating the sitemap information for the identified website by downloading updated sitemap information for the identified website; and scheduling documents for crawling in accordance with the updated sitemap information for the identified website.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/271,160, filed Oct. 11, 2011, now U.S. Pat. No. 8,417,686, which is acontinuation of U.S. patent application Ser. No. 12/823,358, filed Jun.25, 2010, now U.S. Pat. No. 8,037,054, which is a continuation of U.S.patent application Ser. No. 11/172,764, filed Jun. 30, 2005, now U.S.Pat. No. 7,769,742, which claims the benefit of U.S. Provisional PatentApplication No. 60/686,494, filed May 31, 2005, entitled “Web CrawlerScheduler that Utilizes Sitemaps from Websites.”

All above-identified patents and patent applications are herebyincorporated by reference in their entireties.

TECHNICAL FIELD

The disclosed embodiments relate generally to search engines. Moreparticularly, the disclosed embodiments relate to a web crawlerscheduler that utilizes sitemaps from websites.

BACKGROUND

A search engine is a tool that identifies documents, typically stored onhosts distributed over a network, which satisfy search queries specifiedby users. Web search engines work by storing information about a largenumber of documents (such as web pages) which they retrieve from theWorld Wide Web (WWW) via a web crawler. The web crawler follows links(also called hyperlinks) found in crawled documents so as to discoveradditional documents to download. This is also known as discovery-basedcrawling.

Discovery-based crawling has some shortcomings. One shortcoming is thatthe crawl coverage may be incomplete, as there may be documents that thecrawler is not able to discover merely by following links. Also, thecrawler might fail to recognize some links that are embedded in menus,JavaScript scripts, and other web-based application logic, such as formsthat trigger database queries. Another shortcoming is that the crawlermay not know if a document has changed since a prior crawl, and thus maybe skipped during a current crawling cycle. Yet another shortcoming isthat the crawler does not know when it should crawl a particular websiteand how much load to put on the website during the crawling process.Crawling a website during high traffic periods and/or excessive loadduring crawling can deplete network resources from the website,rendering the website less accessible to others.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram of a process for generating a sitemap fora website, according to some embodiments of the present disclosure.

FIG. 2 is a block diagram illustrating a website server, according tosome embodiments of the present disclosure.

FIG. 3 is a block diagram illustrating a data structure for storingsitemap generator control parameters, according to some embodiments ofthe present disclosure.

FIG. 4 is a flow chart illustrating a process for generating a sitemap,according to some embodiments of the present disclosure.

FIG. 5 is a flow chart illustrating another process for generating asitemap, according to some embodiments of the present disclosure.

FIG. 6 is a flowchart illustrating a process for generating adifferential sitemap, according to some embodiments of the presentdisclosure.

FIG. 7 is a block diagram illustrating a web crawler system, accordingto some embodiments of the present disclosure.

FIG. 8 is a block diagram illustrating a sitemap crawler, according tosome embodiments of the present disclosure.

FIG. 9 is a flowchart illustrating a process for scheduling documentdownloads based on information included in a sitemap, according to someembodiments of the present disclosure.

Like reference numerals refer to corresponding parts throughout thedrawings.

SUMMARY

Systems and methods for scheduling documents for crawling are disclosed.In some implementations, a method includes obtaining sitemap informationfor a plurality of websites; and analyzing the sitemap information toidentify a website, in the plurality of websites. The website hassitemap information that is at least potentially out of date. The methodalso includes updating the sitemap information for the identifiedwebsite by downloading updated sitemap information for the identifiedwebsite; and scheduling documents for crawling in accordance with theupdated sitemap information for the identified website.

DESCRIPTION OF EMBODIMENTS

A web server of a website generates a sitemap of URLs (UniversalResource Locators) or URIs (Universal Resource Identifiers) that may becrawled by a web crawler. The sitemap may also include metadataassociated with the URLs and/or URIs included in the sitemaps. The webserver sends a notification to a remote computer associated with a webcrawler. In this context the web server may be considered to be theclient and the web crawler is considered to be the server in aclient-server relationship. The remote computer accesses the sitemap anduses the information included within the sitemap to identify documentsstored at the web server to include in a crawl.

In context of search engines and network crawlers, the term “crawl”means to download and process documents so as to build an index or otherlisting of documents. “A crawl” is an iteration of a crawl process,during which a set of documents are downloaded and processed. The term“crawl” sometimes also means identifying and processing links in thedownloaded documents so as to build a link map or other representationof links between the crawled documents. Typically, the purpose of acrawl is to compile the information in a set of documents for a searchengine, thereby enabling the search engine to identify documentssatisfying search queries.

FIG. 1 is a block diagram illustrating a website 100, according to someembodiments of the present disclosure. The website 100 includes awebsite file system 102, sitemap generator control parameters 104, asitemap generator 106, a sitemap update module 108, a sitemapnotification module 110, sitemaps 114, and sitemap indexes 112. The filesystem 102 may be implemented in some embodiments using any of a numberof file systems, including distributed file systems in which files arestored on multiple computers. In other embodiments, the file system 102may be implemented using a database or search engine that producesdocuments in response to queries. It should be appreciated, however,that FIG. 1 is intended more as functional description of the variousfeatures which may be present in a website than as a structuralschematic of the embodiments described herein. In practice, and asrecognized by those of ordinary skill in the art, items shown separatelycould be combined and some items could be separated. For example, someitems shown separately in FIG. 1 could be implemented on single serversand single items could be implemented by one or more servers. Asrecognized by those of ordinary skill in the art, a website may beimplemented on a single server, such as a web server, or on a pluralityof servers, such as a plurality of web servers. The actual number ofservers used to implement a website server and how features areallocated among them will vary from one implementation to another, andmay depend in part on the amount of data traffic that the system musthandle during peak usage periods as well as during average usageperiods. For convenience of explanation, websites will be describedbelow as if they are implemented on a single web server.

The website file system 102 organizes the documents that are stored atthe web server. A document stored at the website may be anymachine-readable file that includes text, graphics, video, audio, etc.,or any combination thereof. Examples of documents that may be stored ata website include, without limitation, web pages, images, video files,audio files, Portable Document Format (PDF) files, plain text files,executable files, presentation files, spreadsheets, word processordocuments, and so forth.

The documents stored at the website 100 may be organized in a hierarchalstructure. That is, the documents are organized into a tree of nesteddirectories, folders, or paths (hereinafter the “directory tree”). Thedirectory tree includes a root directory/folder/path, and the root mayhave subdirectories/sub-folders/subpaths nested within. Thesubdirectories/sub-folders/subpaths may also have furthersubdirectories/sub-folders/subpaths nested within, thus forming adirectory tree. Each document is stored in a directory/folder/path inthe directory tree. Each directory/folder/path and each document is anode in the tree. The file system also stores metadata associated withthe documents, such as a last modification date, a last access date,document permissions, and the like. In some embodiments, the file systemmay also include a database of documents and associated metadata.Documents in the database may be accessed by executing a query of thedatabase as well as, or instead of, traversing the directory tree.

Each document stored at the website may be identified and/or located bya locator. In some embodiments, the locator is the Uniform ResourceLocator (URL) of the document. In some other documents, alternativemanners of identification (e.g., URIs) or addressing may be used. TheURL of a document may be derived from its location in the file system.The URL of a document may be based on the directory/folder/path, or thelocation in a database, or on the query used to retrieve the documentfrom the database in which the document is stored. That is, eachdocument in a directory/folder/path or a database location may be mappedto an URL. In some embodiments, the URLs may be used by computersexternal to the website, such as remote computers associated with webcrawlers, to access the documents in the file system that are open toexternal access. For convenience of explanation, the document locatorsare described below as if they are URLs.

The sitemap generator 106 generates sitemaps and, optionally, one ormore sitemap indexes of the website. The sitemaps, further details ofwhich are described below, may be used by web crawlers to schedule itscrawls of documents stored at the web server. Sitemap indexes, furtherdetails of which are described below, encapsulate one or more sitemaps.The sitemap index may contain a list of sitemaps. The sitemap generator106 generates sitemaps by accessing one or more sources of documentinformation. In some embodiments, the sources of document informationinclude the file system 102, access logs, pre-made URL lists, andcontent management systems. The sitemap generator may gather documentinformation by simply accessing the website file system 102 andcollecting information about any document found in the file system 102.For instance, the document information may be obtained from a directorystructure that identifies all the files in the file system. The sitemapgenerator 106 may also gather document information by accessing theaccess logs (not shown) of the website. The access logs record accessesof documents by external computers. An access log may include the URLsof the accessed documents, identifiers of the computers accessing thedocuments, and the dates and times of the accesses. The sitemapgenerator 106 may also gather document information by accessing pre-madeURL lists (not shown). The pre-made URL lists list URLs of documentsthat the website operator wishes to be crawled by web crawlers. The URLlists may be made by the website operator using the same format as thatused for sitemaps, as described below.

If the documents in the website are managed via a content managementsystem, the sitemap generator 106 may gather document information byinterfacing with the content management system and accessing theinformation stored within the content management system. Contentmanagement systems are well known in the art and need not be furtherdescribed.

The sitemap generator control parameters 104 include predefinedparameters that control the generation of sitemaps. Further informationregarding the sitemap generator control parameters 104 is describedbelow, in relation to FIG. 3.

The sitemap generator 106 generates sitemaps 114 and possibly one ormore sitemap indexes 112. The sitemap 114 and sitemap index 112 may begenerated using any suitable format and language. In some embodiments,the sitemap is generated in Extensible Markup Language (XML) format,using predefined XML tags. In some other embodiments, other formats maybe used, including but not limited to plain text files, comma-separatedvalue files, and semicolon-separated value files. For convenience ofdescription, sitemaps and sitemap indexes below are described asformatted using XML.

A sitemap index 112 is a document that lists one or more sitemaps 114.The sitemap generator 106, when generating sitemaps for the website, maygenerate multiple sitemaps, with each sitemap listing a subset of URLsof documents that may be crawled, rather than listing URLs of alldocuments that may be crawled in one sitemap. In such a situation, thesitemap generator 106 may also generate a sitemap index 112 to list themultiple sitemaps and their URLs. A sitemap index may include startingand ending tags (e.g., XML tags such as <sitemapindex> and</sitemapindex>, not shown in the Figures) defining the beginning andend of the sitemap index 112. The sitemap index also includes URLs ofeach sitemap listed in the sitemap index. The sitemap index may alsoinclude optional metadata for respective sitemap URLs in the sitemapindex. For instance, the metadata may include a last modification dateof the respective sitemap. Each sitemap URL and any respectiveassociated metadata are enclosed by starting and ending tags that definethe beginning and end of a sitemap record 114 in the sitemap index 112.

In addition to the list of sitemaps, in some embodiments a sitemap indexmay optionally contain a list of site-specific information 140 (alsocalled “per-site information”) that applies to an entire website. Forexample, a sitemap index may contain a list of time intervals and a rateat which the crawler should crawl the respective website (e.g.,

<crawl_rate from=08:00 UTC to =17:00 UTC>medium</crawl_rate>

<crawl_rate from=17:00 UTC to =8:00 UTC>fast</crawl_rate>).

In other examples, a sitemap index contains geographic informationidentifying a geographic location associated with the website (e.g.,<location>latitude, longitude</location>), and/or it may containlanguage information identifying one or more languages supported by orotherwise associated with the respective website (e.g.,<language>German</language>). In some embodiments, per-site informationmay also be present in the sitemaps references in a sitemap index file.If both the sitemap index and a referenced sitemap contain per-siteinformation for the same property (e.g., crawl rate), then the valuespecified in the sitemap overrides the value specified in the sitemapindex. In other embodiments, per-site information may be specified in asitemap index or sitemap using syntax other than the examples givenhere.

In one embodiment, the sitemap generator 106 of a website generates anew sitemap at regular intervals, such as daily or weekly. Each newsitemap generated, after a first (starting point) sitemap, only listsURLs that are new or modified since the prior sitemap was generated(i.e., have a creation date or modification date after the date that thelast sitemap was generated). The term “date” is used here to mean dateand time, and may be represented by a timestamp, such as an ISO 8601compatible timestamp using UTC (coordinated universal time). In theseembodiments, the sitemap index for the website lists all the sitemapsgenerated for the website. Optionally, a new starting point sitemap maybe generated by the sitemap generator at larger intervals (e.g., weeklyor monthly) than the intervals at which the update sitemaps aregenerated. Each time a new sitemap is generated and added to the sitemapindex, a notification is sent to one or more search engines or crawlers.

A sitemap 114 is a document that lists the URLs of documents in awebsite that may be crawled by a web crawler. A sitemap 114 includes alist of URLs and, optionally, additional information, such as metadata,for respective listed URLs. A sitemap 114 may include starting andending tags 116 that define the beginning and end of the sitemap. Thesitemap includes one or more URL records 118. The beginning and end ofeach URL record 118 are defined by a starting tag 120 and an ending tag130. Each URL record 118 includes the URL 122 of a document that may becrawled. A URL record 118 may also include optional metadata associatedwith the respective URL. The optional metadata may include one or moreof the following: a last modification date 124 of the document specifiedby the URL, a change frequency 126 (also called the update rate) of thedocument specified by the URL, a document title 127, a document author129, and a priority 128 of the document specified by the URL. The changefrequency 126 and priority 128 may be specified by the website operator.

The change frequency 126 is a descriptor of how often a document'scontent is expected to change. The descriptor is one of a predefined setof valid descriptors. In some embodiments, the set of change frequencydescriptors includes “always,” “hourly,” “daily,” “weekly,” “monthly,”“yearly,” and “never.” The change frequency 126 provides a hint to thecrawler as to how often the document changes and the crawler can use thehint to schedule crawls of the document accordingly. A crawler, however,may crawl the document in a way that is inconsistent with the specifiedchange frequency. For example, a crawler may crawl a document marked as“hourly” less frequently than a document marked as “yearly.” The actualcrawl frequency of a document may be based on the document's importance(as represented by a score, such as PageRank), actual observed changesin the document (or lack thereof) as observed by the crawler, and otherfactors, as well as the change frequency specified in a sitemap.

The priority 128 is a value that specifies the relative priority of thedocument identified by the URL 122. The priority 128 may be the priorityrelative to other documents listed in the same sitemap 114, relative toother documents stored in the same web server as the document, orrelative to all documents in the website. In some embodiments, the rangeof priority values is 0.0 to 1.0, inclusive, with 0.5 as the defaultvalue, 0.0 as the lowest relative priority, and 1.0 as the highestrelative priority. In some other embodiments, other priority scales,such as 0 to 10, may be used. The priority may be used by a crawler todetermine which documents in the website should be crawled first. Thecrawler may ignore or modify the priority values in a sitemap when thosepriority values fail to meet predefined criteria (e.g., a requirementthat the priority values in a sitemap or set of sitemaps for a websitehave a predefined average value, such as 0.5). In some embodiments, thepriority may also be used when indexing documents.

The sitemap generator 106 may also interact with a sitemap update module108 and a sitemap notification module 110. The sitemap notificationmodule 110 sends a notification to a remote computer associated with aweb crawler whenever a new or updated sitemap is available at thewebsite. The notification includes the URL of the sitemap, so that theremote computer can access the sitemap. If the website uses sitemapindexes, the notification, in some embodiments, includes just the URL ofthe sitemap index. The remote computer accesses the sitemap index andidentifies the URLs of the sitemaps from the sitemap index. In someother embodiments, the notification may include the actual sitemapindex, in which case the remote computer need not access the sitemapindex at the website.

The sitemap update module 108 may generate a differential sitemap basedon a difference between a previously generated sitemap and a currentsitemap. Further information regarding differential sitemaps isdescribed below, in relation to FIG. 6.

FIG. 2 is a block diagram illustrating a website server 200, accordingto some embodiments of the present disclosure. The website server 200(or “web server”) typically includes one or more processing units(CPU's) 202, one or more network or other communications interfaces 204,memory 210, and one or more communication buses or signal lines 212 forinterconnecting these components. The website server 200 optionally mayinclude a user interface 205, which may comprise a display device,mouse, and/or a keyboard. The memory 210 includes high-speed randomaccess memory, such as DRAM, SRAM, DDR RAM or other random access solidstate memory devices; and may include non-volatile memory, such as oneor more magnetic disk storage devices, optical disk storage devices,flash memory devices, or other non-volatile solid state storage devices.Memory 210 may optionally include one or more storage devices remotelylocated from the CPU(s) 202 (e.g., network attached storage). Memory210, or alternately the non-volatile memory device(s) within memory 210,comprises a non-transitory computer readable storage medium. In someembodiments, the memory 210 stores the following programs, modules anddata structures, or a subset thereof:

-   -   an operating system 214 that includes procedures for handling        various basic system services and for performing hardware        dependent tasks;    -   a network communication module 216 that is used for connecting        the website server 200 to other computers via the one or more        communication network interfaces 204 and one or more        communication networks, such as the Internet, other wide area        networks, local area networks, metropolitan area networks, and        so on;    -   a sitemap generation module 106 that generates sitemaps;    -   sitemap control parameters 104 that control or guide sitemap        generation;    -   a sitemap index 112 that lists URLs of sitemaps stored at the        website server 200;    -   one or more sitemap(s) 114 that list URLs of documents that may        be crawled; and    -   a website file system 102 that stores and organizes documents.

Each of the above identified elements may be stored in one or more ofthe previously mentioned memory devices, and corresponds to a set ofinstructions for performing a function described above. The aboveidentified modules or programs (i.e., sets of instructions) need not beimplemented as separate software programs, procedures or modules, andthus various subsets of these modules may be combined or otherwisere-arranged in various embodiments. In some embodiments, memory 210 maystore a subset of the modules and data structures identified above.Furthermore, memory 210 may store additional modules and data structuresnot described above.

Although FIG. 2 shows a “website server,” FIG. 2 is intended more asfunctional description of the various features which may be present in aset of servers than as a structural schematic of the embodimentsdescribed herein. In practice, and as recognized by those of ordinaryskill in the art, items shown separately could be combined and someitems could be separated. For example, some items shown separately inFIG. 2 could be implemented on single servers and single items could beimplemented by one or more servers. The actual number of servers used toimplement a website server and how features are allocated among themwill vary from one implementation to another, and may depend in part onthe amount of data traffic that the system must handle during peak usageperiods as well as during average usage periods.

FIG. 3 is a block diagram illustrating a data structure for storingsitemap generator control parameters, according to some embodiments ofthe present disclosure. The sitemap generator control parameters 104control the generation of sitemaps and sitemap indexes. Each of theparameters may be specified by the operator of the website. Theparameters may include one or more of the following:

-   -   one or more sitemap base URLs 302, which specifies the        location(s) from which the sitemaps may be accessed by remote        computers associated with web crawlers;    -   file path to URL mapping(s) 304, which map        directories/paths/folders or database locations in the file        system 102 to externally accessible URLs (an exemplary path to        URL mapping is P:/A/B/*.*>www.website.com/qu/*.*);    -   URL exclusion pattern(s) 306, which specify classes of URLs that        are to be excluded from inclusion in sitemaps (e.g., an        exclusion pattern of “www.website.com/wa/*.pr1” would indicate        that all “pr1” files in the “/wa” portion of www.website.com are        to be excluded from the sitemap(s));    -   URL pattern(s) with update rates 308, which specify classes of        URLs and update rates (change frequencies) for respective URL        classes (e.g., www.website.com/qu/a*.pdf>daily would indicate        that the files meeting the specified patterns are expected to be        updated daily.    -   notify URL(s) 310, which specify the URLs of remote computers        associated with web crawlers to which new sitemap notifications        may be transmitted;    -   pointer(s) to URL list(s) 312, which point to pre-made URL        lists;    -   pointer(s) to URL access log(s) 314, which point to URL access        logs;    -   pointer(s) to directory or directories 316, which point to        directories/folders/paths or database locations in the file        system 102; and    -   (optional) preferred crawl time(s) 318, which specifies        preferred times of the day for web crawlers to crawl the        website.

It should be appreciated that the parameters listed above are merelyexemplary and that additional and/or alternative parameters may beincluded.

FIG. 4 is a flow chart illustrating a process for generating a sitemap,according to some embodiments of the present disclosure. As describedabove, a source of information about documents stored on the website isthe access logs of the website. The access logs of the website areaccessed (402). The access logs may be found by following pointers 314to URL access logs. The access logs are scanned for non-error URLs(404). Non-error URLs are URLs that properly specify an existing andaccessible document. Thus, for example, a URL for a document that is nolonger on the website may be deemed an error URL. A list of URLs isgenerated (406). The list includes the non-error URLs found in theaccess logs. The list optionally may also include document popularityinformation derived from the access logs. The document popularityinformation may be determined based on the numbers of accesses eachnon-error URL has. The document popularity information serves as anadditional hint of which documents are to be given a higher priorityduring crawling (e.g., scheduled to be crawled first, or more likely tobe crawled than lower priority documents), based on which documents arein high demand (i.e., are accessed more often).

After the list of URLs is generated, the list may optionally be filteredfor excluded URLs (410). The URL exclusion patterns 406 from the sitemapgenerator control parameters 104 may be used as the filter that isapplied to the list of URLs (408). Alternately, URL exclusion patternsmay be obtained elsewhere, or permanently encoded in a customizedsitemap generator for the website. URLs in the list that match any ofthe URL exclusion patterns are removed from the list.

Optional update rate information may be added to the list of URLs, forthe respective URLs in the list (412). In some embodiments, the updaterates may be obtained from the sitemap generator control parameters 104,or more particularly, the URL pattern(s) with update rates 308 (408).

Last modification dates and times for the respective URLs in the list ofURLs are added (414). The last modification dates may be obtained fromthe file system, which may be a database and/or a directory tree 416, asdescribed above.

In an alternate embodiment, a sitemap strategy object 415 controls thefiltering operation 410, update rate information adding operation 412and the last modification date adding operation 414, using informationobtained from a database 416 and/or the sitemap generator controlparameters 408. In some embodiments, the sitemap strategy objectdetermines which URLs (or URIs) to filter and which attributes to add tospecific URLs (or URIs) by performing a database query on the underlyingdatabase 416.

The sitemap is generated from the resulting list of URLs, including anylast modification date/time information, optional popularityinformation, and optional update rate information that has been includedor obtained for the listed URLs (418). Within the sitemap, the metadatafor the URLs listed in the sitemap comprises the last modificationdate/time information, optional popularity information, and optionalupdate rate.

FIG. 5 is a flow chart illustrating another process for generating asitemap, according to some embodiments of the present disclosure. Theprocess of FIG. 5 is similar to that of FIG. 4, with the differencebeing that, in the process of FIG. 5, the initial source of documentinformation is the file system database or directory tree (502), ratherthan access logs. A scan of the database or a traversal of the directorytree is performed (504). From the database scan or directory treetraversal, a list of URLs and associated last modification dates areobtained (506). The list may be optionally filtered for excluded URLs(508), using the URL exclusion patterns from the sitemap generatorcontrol parameters as the filters (512). Optionally, additionalmetadata, such as document update rate information associated with therespective URLs in the list of URLs may be added (510). The update rateinformation may be taken from the sitemap generator control parameters(512). The sitemap is generated from the list of non-excluded URLs, thelast modification date information, and additional information such asthe update rate information.

In an alternate embodiment, a sitemap strategy object 515 controls thefiltering operation 508 and/or adding metadata 510 to the list of URLsor URIs in the sitemap 514 using information obtained from theunderlying database 502 and/or the sitemap generator control parameters512. In some embodiments, the sitemap strategy object 515 determineswhich URLs (or URIs) to filter and which attributes to add to specificURLs (or URIs) by performing a database query on the underlying database502.

It should be appreciated that the sitemap generation processesillustrated in FIGS. 4 and 5 may be adapted to use alternative sourcesof document information and/or use multiple sources of documentinformation. For example, the sitemap generator may initially extractURLs from one or more pre-made URL lists or from a content managementsystem associated with the website. Regardless of the source from whichthe URLs are extracted, the sitemap generator may collect documentmetadata from as many of the sources of document information as needed.For example, the sitemap generator may extract URLs from pre-made URLlists, obtain the last modification dates from the file system andobtain document popularity information from the access logs. Anysuitable combination of sources of document information may be used togenerate the sitemaps.

FIG. 6 is a flowchart illustrating a process for generating adifferential sitemap, according to some embodiments of the presentdisclosure. A differential sitemap is a sitemap that is generated basedon a difference between a previously generated sitemap and a currentsitemap. The differential sitemap includes URLs that were not includedin the previously generated sitemap and URLs that were included in thepreviously generated sitemap but which have new or updated metadata. Forexample, a URL with an updated last modification date would be includedin the differential sitemap. The presence of an updated lastmodification date for the URL means that the document at the respectiveURL has been updated since the previously generated sitemap.

A current sitemap (602) and a previously generated sitemap (604) areprocessed by a differential sitemap generator, such as the sitemapupdate module 108 (606). The differences between the two sitemaps areascertained and a differential sitemap is generated (608).

The sitemaps, if used by web crawlers, may lead to greater crawlcoverage, since the sitemaps may include documents, such as documentsthat are accessible only by a database query, that are not accessible byfollowing links. The sitemaps may also provide last modification dates.Web crawlers may use the last modification dates to determine if adocument has changed, and may thereby avoid crawling documents whosecontents have not changed. The use of sitemaps to avoid crawlingunchanged documents can make web crawlers and network crawlerssignificantly more efficient. The sitemaps also include information fromwhich web crawlers may determine which documents to crawl first and howmuch load to put on the web server during crawling. This, too, may helpconserve network resources.

Web Crawler, SiteMap Crawler, and Scheduler Systems

FIG. 7 is a block diagram illustrating a web crawler system 700,according to some embodiments of the present disclosure. The web crawlersystem 700, which may be a part of and/or associated with a searchengine, crawls locations corresponding to documents stored in webservers. As discussed above, a document stored at the website may be anymachine-readable file that includes text, graphics, video, audio, etc.,or any combination thereof.

The sitemap crawler 705 accesses sitemaps generated by websites or webservers. The sitemap crawler 705 receives sitemap notifications. Thesitemap notifications 730 are received from web servers or websites thathave documents available for crawling. A notification from a web serveror website informs the sitemap crawler that one or more sitemaps listingURLs of documents that may be crawled are available for access. Thenotification may include the URL of a sitemap, or the URLs of two ormore sitemaps. The notification may include the URL of a sitemap index,or it may include the content of a sitemap index. In some embodiments,the notification may include the sitemap index (112, FIG. 1) or the fullsitemap. The sitemap crawler 705 may access the sitemap index at thesitemap index URL to learn the URLs of the sitemaps and then access thesitemaps.

The sitemap crawler 705 accesses sitemaps from web servers or websitesand may store copies of the accessed sitemaps in a sitemap database 732.The sitemap database 732 stores the sitemaps and information associatedwith the sitemaps, such as the web servers and/or websites with whichthe sitemaps are associated, the last modification dates of thesitemaps, and update rate information associated with the sitemaps.

Accessed sitemaps are provided to a sitemap processing module 734 forprocessing. The sitemap processing module 734 processes the sitemaps andidentifies the URLs and associated metadata 736. The sitemaps are asource of URLs and associated metadata information for the URL scheduler702. In some embodiments, an optional, additional source of URLs andassociated metadata is direct submission 703 by users.

The URL scheduler 702 determines which URLs will be crawled in eachcrawling session. The URL scheduler 702 may store that information inone or more data structures (not shown), such as a set of list datastructures. In some embodiments, the URL scheduler 702 allocates URLs tosegments of the data structure, where the segments correspond to thecrawl sessions. In these embodiments, the URL scheduler 702 alsodetermines which URLs within each segment are to be crawled. In someembodiments, there may be a number of URL schedulers 702, which are runprior to each segment being crawled. Each scheduler 702 is coupled to acorresponding URL manager 704, which is responsible for managing thedistribution of URLs to URL server 706. Alternately, each URL scheduler702 may be coupled to two or more of the URL managers, enabling the URLdistribution function for each crawl session to be spread over multipleURL managers. The URL schedulers 702 are adapted to receive URLs andmetadata 736 extracted from sitemaps.

A controller 701 selects a segment for crawling. The selected segment isreferred to hereinafter as the “active segment.” Typically, at the startof each session, the controller 701 selects a different segment as theactive segment so that, over the course of several sessions, all thesegments are selected for crawling in a round-robin manner.

A query-independent score (also called a document score) is computed foreach URL by URL page rankers 722. The page rankers 722 compute a pageimportance score for a given URL. In some embodiments, the pageimportance score is computed by considering not only the number of URLsthat reference a given URL but also the page importance score of suchreferencing URLs. Page importance score data is provided to URL managers704, which pass a page importance score for each URL to URL server 706,robots 708, and content processing servers 710. One example of a pageimportance score is PageRank, which is used the page importance metricused in the Google search engine. An explanation of the computation ofPageRank is found in U.S. Pat. No. 6,285,999, which is incorporated byreference herein in its entirety as background information. In someembodiments, information from the sitemaps may be incorporated into thecomputation of the page importance score. An example of sitemapinformation that may be incorporated into the page importance score isthe priority 128.

From time to time, the URL server 706 requests URLs from the URLmanagers 704. In response, the URL managers 704 provide the URL server706 with URLs obtained from the data structure. The URL server 706 thendistributes URLs from the URL managers 704 to crawlers 708 (hereinafteralso called “robots” or “bots”) to be crawled. A robot 708 is a serverthat retrieves documents at the URLs provided by the URL server 706. Therobots 708 use various known protocols to download pages associated withURLs (e.g., HTTP, HTTPS, Gopher, FTP, etc.). In some embodiments, arobot 708 retrieves from the per-site information database 740 crawlrate and/or crawl interval information for a specified website, and thenuses the retrieved information to control the rate at which URLs or URIsare fetched by the robot 708 from the website.

Pages obtained from URLs that have been crawled by robots 708 aredelivered to content processing servers 710, which perform a number oftasks. In some embodiments, these tasks include indexing the content ofthe pages, generating records of the outbound links in the pages,detecting duplicate pages, and creating various log records to recordinformation about the crawled pages. In one embodiment, these logrecords are stored in log files, including link logs 714, status logs712, and other logs 716. The link logs 714 include a link record foreach document obtained from a URL by a robot 708 and passed to thecontent processing servers 710. Each link log 714 record identifies allthe links (e.g., URLs, also called outbound links) that are found in thedocument associated with the record and the text that surrounds thelink. The information in the link logs 714 is used by the contentprocessing servers 710 to create link maps 720. The records in the linkmap 720 are similar to records in the link logs 714 with the exceptionthat text is stripped and the records are keyed by a “fingerprint” ofthe normalized value of the source URL. In some embodiments, a URLfingerprint is a 64-bit integer determined by applying a hash functionor other one way function to a URL. The bit-length of the URLfingerprint may be longer or shorter than 64 bits in other embodiments.The records in each link map 720 may optionally be sorted or keyed by afingerprint. The link maps 720 are used by the page rankers 722 tocompute or adjust the page importance score of URLs. In someembodiments, such page importance scores may persist between sessions.

The status logs 712 log the status of the document processing performedby the content processing servers 710. The status logs may include URLstatus information 728 (e.g., whether a document existed at thespecified URL, last modification date information, and update rateinformation). The URL status information may be transmitted to the URLscheduler(s) 702. The URL scheduler(s) may use the URL statusinformation to schedule documents for crawling.

In some embodiments, the content processing servers 710 also createanchor maps 718. The anchor maps 718 maps the “anchor text” inhyperlinks to the URL of the target URLs of the hyperlinks. In documentsthat use HTML tags to implement the hyperlinks, the anchor text is thetext located between a pair of anchor tags. For example, the anchor textin the following pair of anchor tags is “Picture of Mount Everest”:

<A href=“http://www.website.com/wa/me.jpg”>Picture of Mount Everest</A>.

In some embodiments, sitemap-supplied document metadata may also be usedfor creating anchor maps. For example, document metadata such as thedocument title 127 (FIG. 1), document author 129, or documentdescription (not shown) may be used to create the anchor maps. However,it should be appreciated that any field appearing in a sitemap may beincluded in an anchor map.

In some embodiments, the records in an anchor map 718 are keyed by thefingerprints of outbound URLs present in link log 714. Thus, each recordin an anchor map 718 comprises the fingerprint of an outbound URL andthe anchor text that corresponds to the URL in link log 714. Anchor maps718 are used by the indexer(s) 724 to facilitate the indexing of “anchortext” as well as to facilitate the indexing of URLs that do not containwords. For example, consider the case in which the target document at anoutbound URL (e.g., the URL in the above example) is a picture of MountEverest and there are no words in the target document. However, anchortext associated with the URL, “Picture of Mount Everest” is included inan index 726, thereby making the target document accessible via a searchengine using the index 726.

The anchor maps 718 and other logs 716 are transmitted to indexer(s)724. The indexer(s) 724 use the anchor maps 718 and other logs 716 togenerate index(es) 726. The index(es) are used by the search engine toidentify documents matching queries entered by users of the searchengine.

FIG. 8 is a block diagram illustrating a sitemap crawler system 800,according to some embodiments of the present disclosure. The sitemapcrawler system 800 typically includes one or more processing units(CPU's) 802, one or more network or other communications interfaces 804,memory 810, and one or more communication buses or signal lines 812 forinterconnecting these components. The sitemap crawler system 800optionally may include a user interface 805, which may comprise akeyboard, mouse, and/or a display device. The memory 810 includeshigh-speed random access memory, such as DRAM, SRAM, DDR RAM or otherrandom access solid state memory devices; and may include non-volatilememory, such as one or more magnetic disk storage devices, optical diskstorage devices, flash memory devices, or other non-volatile solid statestorage devices. Memory 810 may optionally include one or more storagedevices remotely located from the CPU(s) 802. In some embodiments,memory 810 stores the following programs, modules and data structures,or a subset thereof:

-   -   an operating system 814 that includes procedures for handling        various basic system services and for performing hardware        dependent tasks;    -   a network communication module 816 that is used for connecting        the sitemap crawler system 800 to other computers via the one or        more communication network interfaces 804 and one or more        communication networks, such as the Internet, other wide area        networks, local area networks, metropolitan area networks, and        so on;    -   a sitemap database 732 that stores accessed sitemaps;    -   a sitemap crawler 705 that accesses sitemaps provided by web        servers;    -   a sitemap processing module 734 that receives sitemaps and        processes the sitemaps to identify URLs and associated metadata;    -   URL list(s) 818 that list URLs of documents that may be crawled;        and    -   a notification processing module 820 that processes new sitemap        notifications received from web servers.

Each of the above identified elements may be stored in one or more ofthe previously mentioned memory devices, and corresponds to a set ofinstructions for performing a function described above. The aboveidentified modules or programs (i.e., sets of instructions) need not beimplemented as separate software programs, procedures or modules, andthus various subsets of these modules may be combined or otherwisere-arranged in various embodiments. In some embodiments, memory 810 maystore a subset of the modules and data structures identified above.Furthermore, memory 810 may store additional modules and data structuresnot described above.

In embodiments in which one or more of the sitemap indexes or sitemapscontains per-site information, such per-site information is extractedand added to a per-site information database 740 (e.g., by the sitemapcrawler 705). When appropriate information (e.g., language and/orlocation information) is available in the per-site information database740, it is used by the indexers 724 to add per-site information (e.g.,language and/or location information) to the indexes 726. The inclusionof website geography and/or language information in the indexes 726allows searches to be performed by a search engine, using the indexes726, that include geographic and/or language restrictions. For instance,when the indexes of a search engine include geographic information aboutat least some websites, the search engine can service requests such as“pizza within 1 mile of London Bridge, London, England”. When theindexes of a search engine include language information about at leastsome websites, the search engine can service requests such as “GermanURLs containing ‘George Bush’”. In embodiments in which the per-siteinformation includes crawl rate information and/or crawl time intervals,that information is used by the URL schedulers 702 and/or robots 708 tocontrol the times and rates at which web pages are crawled.

Although FIG. 8 shows a “sitemap crawler system,” FIG. 8 is intendedmore as functional description of the various features which may bepresent in a set of servers than as a structural schematic of theembodiments described herein. In practice, and as recognized by those ofordinary skill in the art, items shown separately could be combined andsome items could be separated. For example, some items shown separatelyin FIG. 8 could be implemented on single servers and single items couldbe implemented by one or more servers. The actual number of servers usedto implement a sitemap crawler system and how features are allocatedamong them will vary from one implementation to another, and may dependin part on the amount of data traffic that the system must handle duringpeak usage periods as well as during average usage periods.

FIG. 9 is a flowchart illustrating a process for scheduling documentdownloads based on information included in a sitemap, according to someembodiments of the present disclosure. In some embodiments, schedulingdocuments for downloading means generating a list of documentidentifiers that identify the scheduled documents. The list of documentsidentifiers may be an ordered list, with document identifiers earlier inthe list having higher priority or importance that document identifierslower in the list.

In some embodiments, the sitemap crawler may access a sitemap uponreceipt of a notification that a current version of the sitemap isavailable. Sitemap notifications are received and logged (902). The nextpending sitemap notification is selected (904). The sitemap(s)associated with the selected sitemap notification are downloaded fromthe web server (906).

In some other embodiments, the sitemap crawler may periodically selectsitemaps for processing and access the sitemaps without waiting fornotifications, in addition to or in lieu of waiting for sitemapnotifications. The sitemap database is accessed (908). A sitemap fromthe database is selected for processing (910). The selection may be madebased on information stored in the database, such as last modificationdate information or update rate information. For example, when the “age”of a sitemap (e.g., the current date minus the date of the sitemap, orthe current date minus the date of last modification date in thesitemap), is older than the shortest predicted update period for anydocument listed in the sitemap, the sitemap may be selected fordownloading. The selected sitemap is accessed, by download from the webserver or by accessing the stored copy of the sitemap at the sitemapdatabase (912).

The sitemap database is updated with new sitemap information if suchinformation is received from the download (914). In embodiments in whichone or more of the sitemap indexes or sitemaps contains per-siteinformation, a per-site information database (740, FIG. 7) is updatedwith the received per-site information. For each URL in the sitemap, adetermination as to whether the URL is a candidate for crawling is made(916). The determination may be made based on the URL statusinformation, such as whether the document at the URL has been or islikely to have been updated, or whether the URL properly specifies anaccessible document (924). URLs that are determined to be candidates forcrawling are identified as candidate URLs (926) and each of them isassigned a score (918). The score for each candidate URL may be based onthe page importance score (e.g., PageRank) of the URL and the priorityvalue of the URL, which was extracted from the sitemap. After scoring,the candidate URLs (928) may be run through an optional filter. Thefilter may select a subset of the candidate URLs based on one or morepredefine criteria, such as budgets, site constraints (e.g., limits onthe number of documents that the crawler is allowed to download duringthe time period of the crawl), and so forth. The resulting list ofcandidate URLs is then used to schedule URL downloads (922). As notedabove, scheduling URL downloads may comprise generating an ordered listof URLs or document identifiers, with document identifiers earlier inthe list representing documents having higher priority or importancethat documents placed later in the ordered list. Furthermore, as notedabove, in some embodiments the scheduling operation 922 takes intoaccount per-site information received in the sitemap indexes orsitemaps, such as crawl interval and/or crawl rate information forspecific web sites.

In some embodiments, the scheduler may schedule more documents forcrawling than the crawler can actually crawl. In some embodiments, acrawler may have a crawl budget for a website or web server. The budgetis a maximum number of documents a crawler may crawl, in a particularcrawl session, for the particular website or web server. In other words,the budget is a self-imposed limit, imposed by the web crawler, on howmany documents to crawl for a particular web server or website. Thebudget limits the crawling a crawler will perform for a particularwebsite or web server, ensuring that the crawler can crawl otherwebsites or web servers before reaching its crawl limit.

In some embodiments, a website/web server operator may set siteconstraints to constrain crawling for a particular website or webserver. The goal of the site constraints are to limit crawling for theparticular website or web server, in order to prevent network resourcesassociated with the website or web server from being depleted by thecrawler. The site constraints may include a maximum number of documents,defined by the website operator, which may be crawled for the particularwebsite during a define period of time (e.g., per hour or per day).

It should be appreciated, however, that the predefined criteria forfiltering candidate URLs, as described above, are merely exemplary, andthat additional or alternative criteria may be included.

The filtering of the candidate URLs leads to the generation of a list ofsorted and filtered candidate URLs (930) and optionally a list ofunselected candidate URLs 932. The list of sorted and filtered candidateURLs is transmitted to the scheduler, where the scheduler will schedulethe crawls for the URLs in the list. Optionally, the list of unselectedURLs 932 may be transmitted to a secondary web crawler 934, which mayinclude a secondary scheduler 936. The secondary scheduler 936 schedulesthe URLs in the list 932 for crawling by the secondary web crawler 934.

The URL scheduler may schedule crawls for URLs in the list according tothe document metadata obtained from the sitemaps. As described above,the metadata may include document last modification date information,document update rate information, document priority information, anddocument popularity information.

The scheduler may schedule crawls of URLs based on the last modificationdate information from the sitemaps. The scheduler may defer schedulingof a document corresponding to a URL for crawling if the document hasnot been modified since the last date/time the document was downloadedby the web crawler. In other words, the scheduler may defer schedulingof a document for crawling if the last modification date of the documentis no later than the date/time of the last download of the document bythe web crawler. Such deferral helps conserve network resources byavoiding re-downloading of documents that have not changed.

The scheduler may also schedule crawls of documents based on the updaterate information form the sitemaps. The scheduler may schedule crawls ofa document if a predefined function of the update rate and the lastdate/time at which the document was downloaded satisfy predefinedcriteria. In some embodiments, the document may be scheduled fordownloading if the difference between the last date/time at which thedocument was downloaded and the current time is greater than the updaterate as indicated by the update rate information. For example, if theupdate rate of a document is “weekly” and the last time the document wasdownloaded was two weeks ago, the scheduler may schedule the documentfor downloading. This helps conserve network resources by avoidingdownloading of documents that is expected to be unchanged since the lastdownload.

The scheduler may also adjust scores of candidate URLs based on theirrelative priorities. The scheduler determines boost factorscorresponding to the relative priorities and applies them to the scores.In some embodiments, the scheduler may also determine boost factorsbased on document popularity information, the document popularityinformation being an additional indication of document priority.

In some embodiments, the scores included with the selected or unselectedcandidate URLs may be used to determine which URLs are must-crawl URLs.That is, whether a document is guaranteed to be crawled may be based onthe score. URLs whose scores are high may be designated as must-crawl.This ensures that important pages are scheduled for crawling.

The foregoing description, for purpose of explanation, has beendescribed with reference to specific embodiments. However, theillustrative discussions above are not intended to be exhaustive or tolimit the present disclosure to the precise forms disclosed. Manymodifications and variations are possible in view of the aboveteachings. The embodiments were chosen and described in order to bestexplain the principles of the present disclosure and its practicalapplications, to thereby enable others skilled in the art to bestutilize the present disclosure and various embodiments with variousmodifications as are suited to the particular use contemplated.

What is claimed is:
 1. A method of scheduling documents for crawling,performed on a computer system having one or more processors and memorystoring one or more programs for execution by the one or moreprocessors, the method comprising: obtaining sitemap information for aplurality of websites; analyzing the sitemap information to identify awebsite, in the plurality of websites, having sitemap information thatis at least potentially out of date; updating the sitemap informationfor the identified website by downloading updated sitemap informationfor the identified website; and scheduling documents for crawling inaccordance with the updated sitemap information for the identifiedwebsite; wherein the sitemap information includes one or more sitemapindexes, wherein each sitemap index in the one or more sitemap indicesincludes a list of URLs corresponding to documents stored at a websitein the plurality of websites, and each sitemap index in the one or moresitemap indexes includes information identifying one or more of: a lastmodification date of a URL in the list of URLs, a change frequency of adocument specified by the URL, a title of the document, an authority ofthe document, and a priority of the document.
 2. The method of claim 1,wherein a respective website in the plurality of websites includes oneor more documents, and wherein the one or more documents are storedusing a hierarchical structure.
 3. The method of claim 2, wherein thehierarchical structure is a tree structure.
 4. The method of claim 1,wherein the sitemap information includes one or more sitemaps, whereineach sitemap in the one or more sitemaps is generated using (i) anaccess log, (ii) a pre-made URL list, and (iii) information obtainedfrom a content management system associated with the correspondingwebsite.
 5. The method of claim 4, wherein a sitemap in the one or moresitemaps is in a format selected from the group consisting of an XMLfile, a plain-text file, a comma-separated value file, and asemicolon-separated file.
 6. The method of claim 1, wherein each sitemapindex in the one or more sitemap indexes includes site-specificinformation for the website in the plurality of websites.
 7. The methodof claim 6, wherein the site-specific information includes one or moreof: a list of crawl intervals, a crawl rate, and information identifyinggeographic location of the website.
 8. The method of claim 1, whereinscheduling documents for crawling uses a document's change frequency. 9.The method of claim 1, wherein scheduling documents for crawling isindependent of a document's change frequency.
 10. The method of claim 1,wherein scheduling documents for crawling uses a document's priority.11. The method of claim 1, wherein scheduling documents for crawling isindependent of a document's priority.
 12. The method of claim 1, whereinthe steps recited in claim 1 are executed (i) in response to obtaining anotification of a change to the sitemap information or (ii) inaccordance with a predefined schedule.
 13. The method of claim 12,wherein the notification of a change to the sitemap information includesone or more of: information identifying a sitemap or a sitemap index, ora network location of the sitemap of the sitemap index.
 14. The methodof claim 1, further comprising: generating a sitemap for a website inthe plurality of websites.
 15. The method of claim 14, whereingenerating the sitemap for the website includes: accessing an access logfor the website, the access log having one or more URLs; applying one ormore filters to the one or more URLs, thereby producing one or morefiltered URLs; and generating the sitemap using the one or more filteredURLs.
 16. The method of claim 15, wherein the access log includes anerror URL and a non-error URL.
 17. The method of claim 14, whereingenerating the sitemap for the website includes: performing a databasescan or a directory crawl to obtain one or more URLs associated with thewebsite; applying one or more filters to the one or more URLs, therebyproducing one or more filtered URLs; and generating the sitemap usingthe one or more filtered URLs.
 18. The method of claim 1, whereinsitemap information includes a differential sitemap identifying, for awebsite in the plurality of websites, a difference between a firstsitemap for the website and a second sitemap of the website.
 19. Themethod of claim 18, wherein the first sitemap is a previous sitemap andthe second sitemap is a current sitemap.
 20. The method of claim 18,wherein the differential sitemap includes URLs included in the firstsitemap but not included in the second sitemap.
 21. The method of claim18, wherein scheduling documents for crawling in accordance with theupdated sitemap information uses the differential sitemap.
 22. Themethod of claim 1, wherein the identified website includes a pluralityof URLs, and scheduling documents for crawling in accordance with theupdated sitemap information for the identified website includesscheduling a first URL in the plurality of URLs to be crawled during afirst crawling session and scheduling a second URL in the plurality ofURLs to be crawled during a second crawling session.
 23. The method ofclaim 1, wherein the identified website includes a plurality of URLs,wherein the plurality of URLs is distributed into a plurality of URLsegments, and scheduling documents for crawling in accordance with theupdated sitemap information for the identified website includes, at eachcrawling session, selecting a URL from a different URL segment forcrawl.
 24. The method of claim 1, further comprising generating asitemap for a website in the plurality of websites by a methodcomprising: obtaining one or more URLs associated with the website;indexing content of a webpage at a respective URL in the one or moreURLs; generating records of out-bound links included in the web page;detecting duplicates pages; creating one or more log records for the webpage; and generating the sitemap for the website using the one or morelog records.
 25. The method of claim 24, further comprising generatingan anchor map for the website using anchor text included in a respectiveURL in the one or more URLs.
 26. The method of claim 25, wherein theanchor map includes records keyed by a fingerprint of an out-bound linkincluded in the webpage.
 27. The method of claim 25, further comprisinggenerating search indices for the website, using the anchor map.
 28. Themethod of claim 1, wherein the sitemap information includes one or moresitemaps; and further comprising: selecting a sitemap in the one or moresitemaps for crawling in accordance with one of: a last modificationdate associated with the sitemap and an update rate associated with thesitemap.
 29. The method of claim 1, wherein scheduling documents forcrawling in accordance with the updated sitemap information for theidentified website includes: obtaining a sitemap for the website, thesitemap having a plurality of URLs; selecting one or more candidate URLsfrom the plurality of URLs; assigning a score to each candidate URL inthe one or more candidate URLs; applying one or more filtering criteriato each candidate URL; and scheduling for crawling filtered candidateURLs by a first crawler.
 30. The method of claim 29, further comprisingdistributing a filtered candidate URL or a non-candidate URL in theplurality of URLs, to a second crawler distinct from the first crawler.31. The method of claim 29, wherein the one or more filtering criteriainclude: a crawling budget for the first crawler and one or more siteconstraints for the identified website.
 32. A computer systemcomprising: one or more processors; a memory storing one or moreprograms for execution by the one or more processors, wherein the one ormore programs comprising instructions for: obtaining sitemap informationfor a plurality of websites; analyzing the sitemap information toidentify a website, in the plurality of websites, having sitemapinformation that is at least potentially out of date; updating thesitemap information for the identified website by downloading updatedsitemap information for the identified website; and scheduling documentsfor crawling in accordance with the updated sitemap information for theidentified website; wherein the sitemap information includes one or moresitemap indexes, wherein each sitemap index in the one or more sitemapindices includes a list of URLs corresponding to documents stored at awebsite in the plurality of websites, and each sitemap index in the oneor more sitemap indexes includes information identifying one or more of:a last modification date of a URL in the list of URLs, a changefrequency of a document specified by the URL, a title of the document,an authority of the document, and a priority of the document.
 33. Anon-transitory computer readable storage medium storing one or moreprograms to be executed by a computer system, the one or more programscomprising instructions for: obtaining sitemap information for aplurality of websites; analyzing the sitemap information to identify awebsite, in the plurality of websites, having sitemap information thatis at least potentially out of date; updating the sitemap informationfor the identified website by downloading updated sitemap informationfor the identified website; and scheduling documents for crawling inaccordance with the updated sitemap information for the identifiedwebsite; wherein the sitemap information includes one or more sitemapindexes, wherein each sitemap index in the one or more sitemap indicesincludes a list of URLs corresponding to documents stored at a websitein the plurality of websites, and each sitemap index in the one or moresitemap indexes includes information identifying one or more of: a lastmodification date of a URL in the list of URLs, a change frequency of adocument specified by the URL, a title of the document, an authority ofthe document, and a priority of the document.